Doc: fix outdated wording about parallel seq scans

Doc: fix outdated wording about parallel seq scans

56788d215 adjusted the parallel seq scan code so that instead of handing
out a single block at a time to parallel workers, it now hands out ranges
of blocks.

Here we update the documentation which still claimed that workers received
just 1 block at a time.

Reported-by: Zhang Mingli
Discussion: https://postgr.es/m/17c99615-2c3b-4e4e-9d0b-424a66a7bccd@Spark
Backpatch-through: 14, where 56788d215 was added.